The Yankees have a wealth of catchers, and the Diamondbacks a wealth of shortstops. It makes good economic sense that they make a trade happen.
A basic concept of micro-economic thought is that of marginal utility. In layman's terms: how efficient is a business for each unit of time? In baseball, teams don't use time as their margin but instead use each position to determine if they are extracting the maximum dollar value, much like how a factory would not have two workers doing the job that one can do just as well. The Yankees and Diamondbacks are in such a position, the former with a wealth of catchers, and the latter a glut of shortstops. Two perfect candidates for a trade deal, as has been mentioned before, are John Ryan Murphy (formerly J.R.) and Didi Gregorius. How will these players would be used on their teams now, and how could they be allocated if their teams were swapped?
The Yankees at catcher are solid in their new acquisition Brian McCann, who is projected to provide 3.3 fWAR over 480 plate appearances, with a variety of backstops rounding out another 0.4 fWAR. On their infield end, it's wide open, as is well-documented. Second base, the position that a potential trade target would slot into in 2014, is projected to only produce 1.5 fWAR over the course of the season.
The Diamondbacks are solid at both catcher and shortstop, but they face an issue of marginal utility. At catcher, they have Miguel Montero, who should tabulate about 3.1 fWAR, but should injury strike, Henry Blanco and Tuffy Gosewich are not viable alternatives. At shortstop, Gregorius is likely to win the starting spot, but top prospect Chris Owings is a long-term option who has a much higher ceiling, and Cliff Pennington is a competent backup. Shortstop, total, is projected to put up 2.1 fWAR.
So what happens if they do in fact swap Murphy for Gregorius? For the Yankees, it's an obvious steal. They'd marginally lose nothing at catcher as McCann would still take most of the plate appearances, and at second base, they'd gain about 0.5 fWAR, as Gregorius is projected to be about a 1.6 fWAR player with Dean Anna projected to score about 0.5 fWAR, totaling at 2.1 instead of 1.6. This is not only relevant for 2014 but beyond, when Gregorius could easily slot into Derek Jeter's open spot, a spot that they would now have a cost-controlled and long-term solution to.
On the Diamondbacks' side, it's more of a tough sell, but still an improvement similar to that of the Yankees. Kevin Towers has stated that he wants a "d'Arnaud" level of talent, but with Montero behind the plate, a viable backup is something that could be more valuable. Murphy's projected to put up 1.5 fWAR in 500 plate appearancers; obviously he would not get that many with the Diamondbacks, but that would come out to about 0.7 if he even gets half of that. Montero, getting about 500 plate appearances would still put up 3.0 fWAR, basically gaining the Diamondbacks a little more than half a win. And on shortstop, the Diamondbacks could easily slot in Owings with Pennington as a backup and put up arguably the same fWAR total.
This trade, in my opinion, is a win-win for both teams. The Diamondbacks can clear the way for Owings and provide a stable alternative to Montero, and the Yankees can clear up their catching glut and secure a replacement at second base and for Jeter long-term. And it may seem like splitting hairs that each team could gain a half-win from this trade, but that extra five runs could be crucial in such tight wild card races down the stretch.